The proposal describes a novel, tissue-specific control mechanism by which the product specificity of an enzyme is modified by a second protein; biosynthesis of the modifier protein is likely to be under hormonal control. Evidence is provided which shows that in the lactating mammary gland, the product specificity of the fatty acid synthetase enzyme, is modified by a protein which shifts the product chain-length from long chain to the medium chain fatty acids characteristic of milk fat. The objectives of the proposal are to identify and isolate the chain length modifier protein, to study its physiochemical properties, the mechanism of its interaction with the fatty acid synthetase, and the hormonal regulation of its biosynthesis. Various assay systems are described for detection of the modifier protein, which will be isolated from lactating rat mammary gland using conventional procedures including affinity chromatography. The possibility that the modifier protein acts in an allosteric manner by changing the affinity of the fatty acid synthetase for its substrates will be investigated in a kinetic study. Procedures are described for investigating alternative mechanisms of the modifier protein action such as direct removal of medium chain fatty acids from the enzyme or the specific binding of medium chain fatty acids. Hormonal regulation of synthesis of the modifier protein will be studied in mammary gland explants from pregnant and lactating rats and in cultured cells or explants from glands of pregnant rats.